Method and device for treating oil sludge and oil-containing waste water

ABSTRACT

A mixture of an oil sludge and an oil-contained waste water is first separated into a first oil-contained water and an oily dust containing metal-contained dust, fine metal powder and small fragments. Secondly, the oily dust is washed and separated into a harmless oil-free dust and a second oil-contained water containing the metal. Finally, both the first and second oil-contained water are separated into oil, harmless water and metal hydroxide by electrolysis. Thus, the mixture of the oil sludge and the oil-contained waste water is separated into components, which are in turn recovered, recycled, or converted into new materials, and the purification of the waste water can be realized.

BACKGROUND OF THE INVENTION

The present invention relates to a method and device for treating an oilsludge and an oil-contained waste water which are generated in theprocess of transportation, refining, storage or usage of petroleum.

Generally, industrial waste treating companies are entrusted to treat anoil tank sludge or a crude oil tanker sludge from oil refining companiesin oil producing countries or oil consuming countries, an engine oilfrom machine repairing shops, and a transformer oil from powerdistributing companies. If such an oil sludge is incinerated, a harmfulgas such as SOx, NOx and COx is diffused in the atmosphere to cause acidrain which will damage forest and also cause photochemical smog whichwill lead to infant asthma.

If the oil sludge is thrown away in the ground, a light oil contained inthe oil sludge is vaporized by a solar heat and a ground heat (in caseof desert or soil), and is diffused in the atmosphere to causephotochemical smog. On the other hand, a heavy oil contained in the oilsludge is penetrated into the ground together with rain water to causesoil pollution in a wide area. Further, the heavy oil flows into aground water to cause pollution of the ground water.

The oil in the oil sludge can be recovered by separating the same fromthe water in the oil sludge. However, a process of separating the oilfrom the water in the form of an emulsion is difficult to conduct.

The present inventor has already proposed a sewage treating method anddevice as mentioned below.

In Japanese Patent Laid-open Publication No. 48-59658, a sewage such asdomestic drainage is adjusted to pH (hydrogen ion concentration)7.0±0.5, and is then filtered by zeolite to remove color, odor andmetal. In electrolyzing the pH adjusted liquid, aluminum is used foranode and cathode, and the polarity of both the electrodes is changed toprevent deposition of metal hydroxide onto the electrodes and therebyaccelerate the electrolysis.

In Japanese Patent Laid-open Publication No. 49-32462, a sewage issprayed to contact a blowing purified air, thereby separating oil fromthe sewage and oxidatively deodorizing the same. Then, pH of the sewageis adjusted to 5.5-7.0. Thereafter, the sewage is fed to a plurality ofelectrolytic cells communicated with each other to conduct electrolysisby using aluminum as anode and cathode as changing the polarity of boththe electrodes.

Thus, the sewage such as industrial drainage and domestic drainage canbe treated by the above sewage treating method and device. However, theoil sludge containing 30% or more of heavy oil cannot be treated.

Accordingly, there is a problem that the pollution of the atmosphere andthe ground water cannot be prevented by the above conventional treatingmethod and device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and devicefor treating an oil sludge and an oil-containing waste water whichseparates such an oil sludge into oil, water and dust to be subjected torecovery, recycle or conversion into new materials without incineratingthe oil sludge and without throwing away the same into the ground.

According to one aspect of the present invention, there is provided amethod of treating an oil sludge and an oil-containing waste water,comprising the steps of heating a mixture of said oil sludge and saidoil-containing waste water to give a fluidity thereto; feeding saidmixture to a flashing vessel having an inner pressure maintained at lessthan atmospheric pressure to vaporize off a part of water and a lightoil and obtain a sludge containing an oil-containing dust, heavy oilcomponent and residual water; cooling a vapor of said part of water andsaid light oil to condense said vapor and separate said part of waterfrom said light oil and recovering said part of water and said lightoil; separating said sludge into said oil-containing dust and a primaryoil-containing water comprised of said heavy oil component and saidresidual water by means of a first centrifugal separator; separatingsaid primary oil-containing water into said heavy oil component and saidresidual water as a secondary oil-containing water by means of a secondcentrifugal separator and recovering said heavy oil component; washingsaid oil-containing dust separated by said first centrifugal separatorby using a washing water to elute oil attached to said oil-containingdust into said washing water and thereby obtain an oil-free dust and anemulsion; separating said oil-free dust from said emulsion by means of athird centrifugal separator; washing said oil-free dust by using anacidic water, then washing said oil-free dust by using an alkalinewater, and finally washing said oil-free dust by using a neutral waterto obtain a water-containing dust and a waste washing water; separatingsaid water-containing dust from said waste washing water by means of afourth centrifugal separator to recover said water-containing dust;adding a pH adjusting liquid to a mixture of said secondaryoil-containing water, said emulsion and said waste washing water toadjust a pH value of said mixture; electrolyzing said mixture afteradjustment of the pH value in an electrolytic cell to separate the sameinto a scum containing said emulsion generated at an upper portion ofsaid electrolytic cell, a sedimented substance containing a metalhydroxide generated at a lower portion of said electrolytic cell, and aresidual liquid generated at an intermediate portion of saidelectrolytic cell; returning said scum to said second centrifugalseparator to treat the same; recovering said metal hydroxide; anddischarging said residual liquid.

According to another aspect of the present invention, there is provideda device for treating an oil sludge and an oil-contained waste water,comprising a first heat exchanger for heating a mixture of said oilsludge and said oil-containing waste water to give a fluidity thereto; aflashing vessel for vaporizing a part of water and a light oil containedin said mixture under a low vacuum; a second heat exchanger for coolinga vapor of said part of water and said light oil to condense said vaporand separate said part of water from said light oil; a first centrifugalseparator for separating a sludge left in said flashing vessel and aprimary oil-containing water comprised of a heavy oil component and aresidual water; a second centrifugal separator for separating saidprimary oil-containing water into said heavy oil component and saidresidual water as a secondary oil-containing water; a first washingwater for washing said oil-containing dust separated by said firstcentrifugal separator to elute oil attached to said oil-containing dustinto said washing water and thereby obtain an oil-free dust and anemulsion; a third centrifugal separator for separating said oil-freedust from said emulsion; a second washing water for washing saidoil-free dust to obtain a water-containing dust and a waste washingwater, said second washing water being comprised of an acidic water, analkaline water and a neutral water; a fourth centrifugal separator forseparating said water-containing dust from said waste washing water; apH adjusting liquid for adjusting a pH value of a mixture of saidsecondary oil-containing water, said emulsion and said waste washingwater; an electrolytic cell for electrolyzing said mixture afteradjustment of the pH value to generate a hydrogen gas at a cathode andan oxygen gas and a metal hydroxide at an anode; and a dehydrator fordehydrating said metal hydroxide.

In summary, a mixture of an oil sludge and an oil-contained waste wateris first separated into a first oil-containing water and an oily dustcontaining metal-contained dust, fine metal powder and small fragments.Secondly, the oily dust is washed and separated into a harmless oil-freedust and a second oil-containing water containing the metal. Finally,both the first and second oil-containing water are separated into oil,harmless water and metal hydroxide by electrolysis. Thus, the mixture ofthe oil sludge and the oil-containing waste water is separated intocomponents, which are in turn recovered, recycled, or converted into newmaterials, and the purification of the waste water can be realized.

Other objects and features of the invention will be more fullyunderstood from the following detailed description and appended claimswhen taken with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B flow diagrams showing the first stage of the treatmentprocess according to the present invention;

FIG. 2 is a flow diagram showing the second stage of the treatmentprocess according to the present invention; and

FIG. 3 is a flow diagram showing the third stage of the treatmentprocess according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Generally, an oil tank sludge in a crude oil tank (including A-oil,B-oil and C-oil) has the following components.

oil: 30-35%

water: 50-40%

dust: 20-25%.

Further, a crude oil tanker sludge has the following components.

oil: 40-45%

water: 30-25%

dust: 20%.

These tank sludge and tanker sludge have a transparency of about 2 cm orless.

Such a sludge is a blackish brown dense solution containing anoil-containing water and dust mixed with each other.

The dust contains a fine quantity of metals such as iron, cadmium,manganese, magnesium and lead. These metals are taken with oil from anoil well.

A waste oil is classified into an engine oil and a transformer oil. Theengine oil to be discharged upon oil exchange contains water and finemetal powder generated due to wear of a cylinder. On the other hand, thetransformer oil contains water and small fragments of a solid adhesive.

In the preferred embodiment, the oil tank sludge, the oil tanker sludgeand the waste oil will be referred generally to as an oil sludge.

The treatment process of the present invention is generally comprised ofthe three stages of first separating the oil sludge into a firstoil-contained water and an oily dust containing metal-containing dust,fine metal powder and small fragments as mentioned above, secondlywashing the above oily dust to separate the same into a harmlessoil-free dust and a second oil-containing water containing the abovemetal, and finally separating both the first and second oil-containingwater into oil, harmless water and metal hydroxide.

There will now be described a preferred embodiment of the presentinvention with reference to the drawings. Referring to FIG. 1 whichshows the first stage of the treatment process according to the presentinvention, an oil tank sludge 1 and an oil tanker sludge 2 are throwninto a primary tank 3 as a recovery oil tank, and they are heated bysteam S at 40°-60° C. to reduce viscosity of the primary oil sludge andgive a fluidity thereto.

The primary oil sludge is fed onto a vibrating screen 4 by a pump toseparate off a rough dust having a particle size of 5 to 1 mm. Asecondary oil sludge having passed through the vibrating screen 4 is fedto a waste oil tank 5. The oily dust left on the vibrating screen 4 isfed to an oily dust tank 20 connected to a dust washing line which willbe hereinafter described.

On the other hand, an engine oil 6 and a transformer oil 7 are alsothrown into the waste oil tank 5, and they are heated by steam S at40°-60° C. to reduce the viscosity of the waste oil and give a fluiditythereto. In the waste oil tank 5, the secondary oil sludge and the wasteoil are blended to obtain a fluidic blend sludge. The blend sludge isfed to a first heat exchanger 8 for heating. The first heat exchanger 8is supplied with steam S to heat the blend sludge at an inlettemperature of 40°-45° C. and at an outlet temperature of 60°-90° C.After passing through the first heat exchanger 8, the blend sludge isfed through a pipe to a flashing vessel 9.

An inner pressure of the flashing vessel 9 is maintained under a lowvacuum, e.g., 300-500 mmHg by using an external vacuum pump (not shown).Accordingly, a volatile light oil vapor (gasoline) and a part of watervapor in the blend sludge fed to the flushing vessel 9 are fed throughan exhaust pipe to a second heat exchanger 11 for cooling. The secondheat exchanger 11 is cooled by a flowing water having a temperature of20°-18° C., and the light oil vapor and the water vapor are thereforecondense. Thus, the light oil (gasoline) and the water are stored at anupper portion and a lower portion of the second heat exchanger 11,respectively. The light oil at the upper portion is fed by gravity to alight oil tank 10, while the water at the lower portion is fed to awater reservoir 45 for the purpose of recycling or discharging.

The residual viscous oily dust having an increased content of dustcollected in bottom portion of the flashing vessel 9 is fed as atertiary oil sludge to a first decanter 12 as the first centrifugalseparator of the present invention by using a pump. The first decanter12 is rotated at a low speed of 3,000-5,000 RPM to separate the tertiaryoil sludge into a primary oil-containing water and an oily dust having asmall particle size in accordance with a difference in specific gravityby a centrifugal force. At this time, since the oily dust having arelatively large particle size has been already separated off by thevibrating screen 4, there is no possibility that the scattering oilydust in the first decanter 12 will damage an inner wall of the firstdecanter 12. The oily dust is almost separated off by sedimentation inthe first decanter 12, and it is then fed to the oily dust tank 20. Onthe other hand, the residual primary oil-containing water is fed to avessel 13 as the second centifugal separator of the present invention.The vessel 13 is rotated at a high speed of 15,000-36,000 RPM toseparate the primary oil-containing water into a heavy oil and asecondary oil-containing water containing 21% of oil. The heavy oil isfed as a regenerative oil to an oil reservoir 30, while the secondaryoil-contained water is fed to an oil-containing water tank 40 connectedto a water treatment line which will be hereinafter described.

The oily dust collected in the oily dust tank 20 contains 10-15% byweight of oil, and it is accordingly fed to the dust washing line asshown in FIG. 2.

Referring to FIG. 2 which shows the second stage of the treatmentprocess according to the present invention, the oily dust is fed fromthe oily dust tank 20 to a first washing tank 21, and 70% by weight of aSKH wash 22 is added to 30% by weight of the oily dust in the firstwashing tank 21, thereby washing the oily dust with stirring. The SKH 22is a solvent having a hydrophilic group which is readily bonded to waterand a lipophilic group which is readily bonded to oil. Therefore, theoil attached to the dust is bonded to the lipophilic group by washingthe oily dust with the SKH 22, and the oil content in the dust isreduced to a trace of 0.01% or less. Thus, the oil attached to the dustsubstantially completely migrates to the SKH 22 to form an emulsion.Then, a mixture of the oil-free dust and the emulsion as obtained aboveis separated into the oil-free dust and the emulsion by a seconddecanter 23 as the third centrifugal separator of the present invention.The emulsion is fed to the oil-contained water tank 40, while theoil-free dust is fed to an oil-free dust tank 24.

The oil-free dust is fed from the oil-free dust tank 24 to a secondwashing tank 25, and a mixed acid water 26 is first added to theoil-free dust in the second washing tank 25 to wash the same withstirring. Then, a mixture of the acid water 26 and the oil-free dust isseparated into a waste acidic water and a primary washed dust by a thirddecanter 27. The waste acidic water is fed to the oil-containing watertank 40, while the primary washed dust is returned to the second washingtank 25.

Secondly, an alkali water 28 is added to the primary washed dustreturned to the second washing tank 25 to wash the same with stirring.Then, a mixture of the alkali water 28 and the primary washed dust isseparated into a waste alkaline water and a secondary washed dust by thethird decanter 27. The waste alkaline water is fed to the oil-containingwater tank 40, while the secondary washed dust is returned to the secondwashing tank 25.

Finally, a neutral water 29 is added to the secondary washed dustreturned to the second washing tank 25 to wash the same with stirring.Then, a mixture of the neutral water 29 and the secondary washed dust isseparated into a waste neutral water and a tertiary washed dust by thethird decanter 27. The waste neutral water is fed to the oil-containingwater tank 40, while the tertiary washed dust is fed to a drying line301.

The tertiary washed dust is dehydrated to a water content of 30% or lessin the drying line 301. This dust may be thrown away on open land orused for reclaiming since the dust finally obtained is harmless.

After the secondary oil-containing water as shown in FIG. 1, theemulsion, the waste acidic water, the waste alkaline water and the wasteneutral water (these waste waters will been hereinafter referredgenerally to as a waste washing water) as shown in FIG. 2 are fed to theoil-containing water tank 40, they are fed to the water treatment lineas shown in FIG. 3.

Referring to FIG. 3 which shows the third stage of the treatment processaccording to the present invention, a mixture of the secondaryoil-containing water, the emulsion and the waste washing water in theoil-containing water tank 40 is fed to a pH adjusting tank 41, and a pHadjusting liquid 42 is added to the mixture in the pH adjusting tank 41as stirring to adjust a pH value of the mixture to a predeterminedvalue. After adjustment of the pH value, the mixture is allowed to standfor a while.

The above mixture of the secondary oil-containing water, the emulsionand the waste washing water contains various metals, which have beenpresent in the original dust, such as antimony Sb, arsenic As, bariumBa, beryllium Be, cadmium Cd, chromium Cr(VI), Cr(III), cobalt Co,copper Cu, lead Pb, mercury Hg, molybdenum Mo, nickel Ni, selenium Se,silver Ag, thallium Tl, vanadium V and zinc Zn.

These metals are combined with OH in the above mixture and the pHadjusting liquid to form a metal hydroxide having a coagulating ability.The metal hydroxide coagulates fine impurities such as organic substanceand inorganic substance still finely residing in the above mixture toform a coagulated substance of the metal hydroxide and the fineimpurities. The coagulated substance is sedimented at a lower portion ofthe pH adjusting tank 41.

The coagulated substance containing the metal hydroxide and the pHadjusting liquid is fed from the bottom of the pH adjusting tank 41 toan electrolytic cell 43. In the electrolytic cell 43, water-solublealuminum Al is used for a cathode and an anode, and a voltage of DC 3-10V is applied. At the cathode, a hydrogen gas is generated in accordancewith the reaction of H₂ O⃡H⁺ +(OH)⁻, 2H⁺ +2e⁻ →H₂. The bonding betweenthe SKH 22 and the oil of the emulsion is cut by a part of the hydrogengas, and the residual hydrogen gas is attached to the oil to float thesame up to an upper portion of the electrolytic cell 43, thus forming ascum. The scum is fed to the vessel 13 as the second centrifugalseparator shown in FIG. 1, and is separated into oil and water by thevessel 13.

At the anode of the electrolytic cell 43, Al³⁺ is eluted from the Alelectrode plate to react with (OH)⁻ present in the electrolyte and forman aluminum hydroxide in accordance with the reaction of Al³⁺ +3(OH)⁻→Al(OH)₃.

The aluminum hydroxide serves to a coagulant to coagulate fineimpurities such as organic substance and inorganic substance stillfinely residing in the electrolyte to form a coagulated substance. Thecoagulated substance is sedimented at a lower portion of theelectrolytic cell 43. Simultaneously, the coagulated substancecontaining the metal hydroxide fed from the pH adjusting tank 41 is alsosedimented at the lower portion of the electrolytic cell 43. Then, thesecoagulated substances containing the aluminum hydroxide and the metalhydroxide are fed from the electrolytic cell 43 to a dehydrator 44.

A liquid present at an intermediate layer between the upper layer of thescum and the lower layer of the coagulated substances in theelectrolytic cell 43 is a substantially clarified water, and the liquidis fed to the water reservoir 45.

In the dehydrator 44, the coagulated substances containing the aluminumhydroxide and the metal hydroxide are dehydrated and separated intowater and dehydrated cake. The water is fed to the water reservoir 45,while the dehydrated cake is fed to a drier 46. In the drier 46, thedehydrated cake is dried to a water content of 30% or less, thus formingmetal oxide, which is subjected to recycling of metal.

Although the water in the water reservoir 45 has been already clarified,there is a fear that a part of the water residing at the lower portionof the water reservoir 45 contains a small amount of sludge. Therefore,the water is fed again to the oil-contained water tank 40 of the watertreatment line.

Before the treatment by the water treatment line, the oil-containedwater in the oil-containing water tank 40 has COD of 12,000-5,000 ppm,BOD of 8,500-3,500 ppm, SS of 1,000-500 ppm, n-Hex extract (oil) of2,000-1,000 ppm, and transparency of 25-35 cm. In contrast, after thetreatment, the water to be discharged from the water reservoir 45 hasCOD of 20 ppm or less, BOD of 27-68 ppm, SS of 10 ppm or less, n-Hexextract (oil) of 5 ppm or less, pH of 7±1, transparency of 100 cm ormore, and E. coli of trace.

Thus, the water to be discharged from the water reservoir 45 isharmless, and it is accordingly discharged to a river or subjected torecycling (the neutral washing water for the dust). The oil recovered isreused.

While the invention has been described with reference to a specificembodiment, the description is illustrative and is not to be construedas limiting the scope of the invention. Various modifications andchanges may occur to those skilled in the art without departing from thespirit and scope of the invention as defined by the appended claims.

What is claimed is:
 1. A method of treating an oil sludge and anoil-containing waste water, comprising the steps of heating a mixture ofsaid oil sludge and said oil-containing waste water to give a fluiditythereto; feeding said mixture to a flashing vessel having an innerpressure maintained less than atmospheric pressure to vaporize off apart of the water and a light oil and to obtain a sludge containing anoil-containing dust, a heavy oil component and residual water; cooling avapor of said part of water and said light oil to condense said vaporand separate said part of water from said light oil and recovering saidpart of water and said light oil; separating said sludge into saidoil-containing dust and a primary oil-containing water comprised of saidheavy oil component and said residual water by means of a firstcentrifugal separator; separating said primary oil-containing water intosaid heavy oil component and said residual water as a secondaryoil-containing water by means of a second centrifugal separator andrecovering said heavy oil component; washing said oil-containing dustseparated by said first centrifugal separator with a washing liquid toelute oil attached to said oil-containing dust into said washing waterand thereby obtain an oil-free dust and an emulsion; separating saidoil-free dust from said emulsion by means of a third centrifugalseparator; washing said oil-free dust with an acidic water, then washingsaid oil-free dust with an alkaline water, and finally washing saidoil-free dust with a neutral water to obtain a waste washing watercontaining said oil-free dust; separating said dust from said wastewashing water by means of a fourth centrifugal separator to recover saiddust; adding a pH adjusting liquid to a mixture of said secondaryoil-containing water, said emulsion and said waste washing water toadjust a pH value of said mixture; electrolyzing said mixture afteradjustment of the pH value in an electrolytic cell to separate the sameinto a scum containing said emulsion generated at an upper portion ofsaid electrolytic cell, a sedimented substance containing a metalhydroxide collected at a lower portion of said electrolytic cell, and aresidual liquid generated at an intermediate portion of saidelectrolytic cell; returning said scum to said second centrifugalseparator to treat the same; recovering said metal hydroxide; anddischarging said residual liquid.
 2. The method as defined in claim 1further comprising the step of separating off a part of saidoil-containing dust by means of a vibrating screen before feeding saidmixture of said oil sludge and said oil-containing waste water to saidflashing vessel.
 3. The method as defined in claim 1, wherein said innerpressure of said flushing vessel is maintained in the range of 300-500mmHg.
 4. The method as defined in claim 1, wherein said heating step iscarried out by means of steam.
 5. The method as defined in claim 1,wherein said first centrifugal separator is rotated at a low speed of3,000-5,000 RPM.
 6. The method as defined in claim 1, wherein saidsecond centrifugal separator is rotated at a high speed of 15,000-36,000RPM.
 7. The method as defined in claim 1, wherein said washing liquidcontains a solvent having a hydrophilic group and a lipophilic group. 8.The method as defined in claim 1, wherein said oil-free dust washed byusing said acidic water is dehydrated by said fourth centrifugalseparator.
 9. The method as defined in claim 1, wherein said oil-freedust washed by using said alkaline water is dehydrated by said fourthcentrifugal separator.
 10. The method as defined in claim 1, whereinsaid oil-free dust washed by using said neutral water is dehydrated bysaid fourth centrifugal separator.
 11. The method as defined in claim 1,wherein said scum returned to said second centrifugal separator isseparated into oil and water.
 12. The method as defined in claim 1further comprising the step of dehydrating said sedimented substancecontaining said metal hydroxide by means of a dehydrator before saidrecovering step of said metal hydroxide.